[PATCH 3/4] x86/dumpstack: remove raw stack dump

Josh Poimboeuf
2016-10-25T14:51:28Z

For mostly historical reasons, the x86 oops dump shows the raw stack
values:

...
[registers]
Stack:
ffff880079af7350 ffff880079905400 0000000000000000 ffffc900008f3ae0
ffffffffa0196610 0000000000000001 00010000ffffffff 0000000087654321
0000000000000002 0000000000000000 0000000000000000 0000000000000000
Call Trace:
...

This seems to be an artifact from long ago, and probably isn't needed
anymore. It generally just adds noise to the dump, and it can be
actively harmful because it leaks kernel addresses.

Linus says:

"The stack dump actually goes back to forever, and it used to be
useful back in 1992 or so. But it used to be useful mainly because
stacks were simpler and we didn't have very good call traces anyway. I
definitely remember having used them - I just do not remember having
used them in the last ten+ years.

Of course, it's still true that if you can trigger an oops, you've
likely already lost the security game, but since the stack dump is so
useless, let's aim to just remove it and make games like the above
harder."

This also removes the related 'kstack=' cmdline option and the
'kstack_depth_to_print' sysctl.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
Documentation/kernel-parameters.txt | 3 --
Documentation/sysctl/kernel.txt | 8 -----
Documentation/x86/x86_64/boot-options.txt | 4 ---
arch/x86/include/asm/stacktrace.h | 5 ---
arch/x86/kernel/dumpstack.c | 21 ++----------
arch/x86/kernel/dumpstack_32.c | 33 +------------------
arch/x86/kernel/dumpstack_64.c | 53 +------------------------------
kernel/sysctl.c | 7 ----
8 files changed, 4 insertions(+), 130 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 73de01e..def1874 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1952,9 +1952,6 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
kmemcheck=2 (one-shot mode)
Default: 2 (one-shot mode)

- kstack=N [X86] Print N words from the kernel stack
- in oops dumps.
-
kvm.ignore_msrs=[KVM] Ignore guest accesses to unhandled MSRs.
Default is 0 (don't ignore, but inject #GP)

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index ffab8b5..065f184 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -40,7 +40,6 @@ show up in /proc/sys/kernel:
- hung_task_warnings
- kexec_load_disabled
- kptr_restrict
-- kstack_depth_to_print [ X86 only ]
- l2cr [ PPC only ]
- modprobe ==> Documentation/debugging-modules.txt
- modules_disabled
@@ -395,13 +394,6 @@ When kptr_restrict is set to (2), kernel pointers printed using

==============================================================

-kstack_depth_to_print: (X86 only)
-
-Controls the number of words to print when dumping the raw
-kernel stack.
-
-==============================================================
-
l2cr: (PPC only)

This flag controls the L2 cache of G3 processor boards. If
diff --git a/Documentation/x86/x86_64/boot-options.txt b/Documentation/x86/x86_64/boot-options.txt
index 0965a71..61b611e 100644
--- a/Documentation/x86/x86_64/boot-options.txt
+++ b/Documentation/x86/x86_64/boot-options.txt
@@ -277,10 +277,6 @@ IOMMU (input/output memory management unit)
space might stop working. Use this option if you have devices that
are accessed from userspace directly on some PCI host bridge.

-Debugging
-
- kstack=N Print N words from the kernel stack in oops dumps.
-
Miscellaneous

nogbpages
diff --git a/arch/x86/include/asm/stacktrace.h b/arch/x86/include/asm/stacktrace.h
index 37f2e0b..1e375b0 100644
--- a/arch/x86/include/asm/stacktrace.h
+++ b/arch/x86/include/asm/stacktrace.h
@@ -43,8 +43,6 @@ static inline bool on_stack(struct stack_info *info, void *addr, size_t len)
addr + len > begin && addr + len <= end);
}

-extern int kstack_depth_to_print;
-
#ifdef CONFIG_X86_32
#define STACKSLOTS_PER_LINE 8
#else
@@ -86,9 +84,6 @@ get_stack_pointer(struct task_struct *task, struct pt_regs *regs)
void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
unsigned long *stack, char *log_lvl);

-void show_stack_log_lvl(struct task_struct *task, struct pt_regs *regs,
- unsigned long *sp, char *log_lvl);
-
extern unsigned int code_bytes;

/* The form of the top of the frame on the stack */
diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index f967652..499aa6f 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -22,7 +22,6 @@
int panic_on_unrecovered_nmi;
int panic_on_io_nmi;
unsigned int code_bytes = 64;
-int kstack_depth_to_print = 3 * STACKSLOTS_PER_LINE;
static int die_counter;

bool in_task_stack(unsigned long *stack, struct task_struct *task,
@@ -171,12 +170,12 @@ void show_stack(struct task_struct *task, unsigned long *sp)
if (!sp && task == current)
sp = get_stack_pointer(current, NULL);

- show_stack_log_lvl(task, NULL, sp, KERN_DEFAULT);
+ show_trace_log_lvl(task, NULL, sp, KERN_DEFAULT);
}

void show_stack_regs(struct pt_regs *regs)
{
- show_stack_log_lvl(current, regs, NULL, KERN_DEFAULT);
+ show_trace_log_lvl(current, regs, NULL, KERN_DEFAULT);
}

static arch_spinlock_t die_lock = __ARCH_SPIN_LOCK_UNLOCKED;
@@ -295,22 +294,6 @@ void die(const char *str, struct pt_regs *regs, long err)
oops_end(flags, regs, sig);
}

-static int __init kstack_setup(char *s)
-{
- ssize_t ret;
- unsigned long val;
-
- if (!s)
- return -EINVAL;
-
- ret = kstrtoul(s, 0, &val);
- if (ret)
- return ret;
- kstack_depth_to_print = val;
- return 0;
-}
-early_param("kstack", kstack_setup);
-
static int __init code_bytes_setup(char *s)
{
ssize_t ret;
diff --git a/arch/x86/kernel/dumpstack_32.c b/arch/x86/kernel/dumpstack_32.c
index 06eb322..90cf460 100644
--- a/arch/x86/kernel/dumpstack_32.c
+++ b/arch/x86/kernel/dumpstack_32.c
@@ -121,36 +121,6 @@ int get_stack_info(unsigned long *stack, struct task_struct *task,
return -EINVAL;
}

-void show_stack_log_lvl(struct task_struct *task, struct pt_regs *regs,
- unsigned long *sp, char *log_lvl)
-{
- unsigned long *stack;
- int i;
-
- if (!try_get_task_stack(task))
- return;
-
- sp = sp ? : get_stack_pointer(task, regs);
-
- stack = sp;
- for (i = 0; i < kstack_depth_to_print; i++) {
- if (kstack_end(stack))
- break;
- if ((i % STACKSLOTS_PER_LINE) == 0) {
- if (i != 0)
- pr_cont("\n");
- printk("%s %08lx", log_lvl, *stack++);
- } else
- pr_cont(" %08lx", *stack++);
- touch_nmi_watchdog();
- }
- pr_cont("\n");
- show_trace_log_lvl(task, regs, sp, log_lvl);
-
- put_task_stack(task);
-}
-
-
void show_regs(struct pt_regs *regs)
{
int i;
@@ -168,8 +138,7 @@ void show_regs(struct pt_regs *regs)
unsigned char c;
u8 *ip;

- pr_emerg("Stack:\n");
- show_stack_log_lvl(current, regs, NULL, KERN_EMERG);
+ show_trace_log_lvl(current, regs, NULL, KERN_EMERG);

pr_emerg("Code:");

diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
index 36cf1a4..310abf4 100644
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -140,56 +140,6 @@ int get_stack_info(unsigned long *stack, struct task_struct *task,
return -EINVAL;
}

-void show_stack_log_lvl(struct task_struct *task, struct pt_regs *regs,
- unsigned long *sp, char *log_lvl)
-{
- unsigned long *irq_stack_end;
- unsigned long *irq_stack;
- unsigned long *stack;
- int i;
-
- if (!try_get_task_stack(task))
- return;
-
- irq_stack_end = (unsigned long *)this_cpu_read(irq_stack_ptr);
- irq_stack = irq_stack_end - (IRQ_STACK_SIZE / sizeof(long));
-
- sp = sp ? : get_stack_pointer(task, regs);
-
- stack = sp;
- for (i = 0; i < kstack_depth_to_print; i++) {
- unsigned long word;
-
- if (stack >= irq_stack && stack <= irq_stack_end) {
- if (stack == irq_stack_end) {
- stack = (unsigned long *) (irq_stack_end[-1]);
- pr_cont(" <EOI> ");
- }
- } else {
- if (kstack_end(stack))
- break;
- }
-
- if (probe_kernel_address(stack, word))
- break;
-
- if ((i % STACKSLOTS_PER_LINE) == 0) {
- if (i != 0)
- pr_cont("\n");
- printk("%s %016lx", log_lvl, word);
- } else
- pr_cont(" %016lx", word);
-
- stack++;
- touch_nmi_watchdog();
- }
-
- pr_cont("\n");
- show_trace_log_lvl(task, regs, sp, log_lvl);
-
- put_task_stack(task);
-}
-
void show_regs(struct pt_regs *regs)
{
int i;
@@ -207,8 +157,7 @@ void show_regs(struct pt_regs *regs)
unsigned char c;
u8 *ip;

- printk(KERN_DEFAULT "Stack:\n");
- show_stack_log_lvl(current, regs, NULL, KERN_DEFAULT);
+ show_trace_log_lvl(current, regs, NULL, KERN_DEFAULT);

printk(KERN_DEFAULT "Code: ");

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 739fb17..39b3368 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -983,13 +983,6 @@ static struct ctl_table kern_table[] = {
.proc_handler = proc_dointvec,
},
{
- .procname = "kstack_depth_to_print",
- .data = &kstack_depth_to_print,
- .maxlen = sizeof(int),
- .mode = 0644,
- .proc_handler = proc_dointvec,
- },
- {
.procname = "io_delay_type",
.data = &io_delay_type,
.maxlen = sizeof(int),
--
2.7.4